I'm on the corporate that's why I can get 18%.
For non corp customers, the best credit card to charge your
petrol expenditure to would be Citibank Dividend.
You get the standard 5% on site discount plus an additional 5%
rebate to your credit card a/c.
If you pump at exxon, there's another additional 3% rebate to
your credit card.
With the Dividend Card, Citibank will send you a cheque at the
end of every three months on all the rebates accumulated.
Which means bros here who spend like $500 on petrol after discount,
effectively you would be having a $40 cheque sent to you, not to
mention that it would be more given that you would have rebates at
every transaction point:)
A little long post early in the morning but and I do hope it's
not side tracking of this topic 
